Negative Feedback Equation

Discussion in 'Homework Help' started by cambria83, Nov 14, 2007.

  1. cambria83

    Thread Starter New Member

    Nov 14, 2007
    3
    0
    Hi,

    I am really stuck with a negative feedback equation! I am learning the basics of op-amps on my music tech. course!

    If I was to calculate the output voltage of the closed loop gain circuit when the input voltage is 14mV and the negative feedback voltage is 4mV where would I start?

    This is probably really simple stuff but I am a complete newbie to all this and finding it a bit hard to get my head around at the moment!

    Thanks for any help you can provide!

    -Neil-
     
  2. Management

    Active Member

    Sep 18, 2007
    306
    0
    Depending on the configuration of your feedback and if it is inverting or non-inverting you will get different equations. Try reading this: http://www.allaboutcircuits.com/vol_3/chpt_8/4.html

    Is that 4mV feedback voltage coming from the output of your amplifier (I am assuming)? Do you have a schematic?
     
  3. cambria83

    Thread Starter New Member

    Nov 14, 2007
    3
    0
    Hi,

    Thanks for the reply! I did read that article earlier but it didn't really help me to understand how to work out the output voltage from the information I have!

    As far as I know it is inverting! The exact question I have here is:

    Using the negative feedback equation, calculate the output voltage (Vout) when input voltage (Vin) is 14mV and negative feedback voltage is 4mV.

    I have just written a 1500 report on negative feedback but I still don't get how to work this out! I've been trying to find equations to work it out for hours!

    Is this any help?
     
  4. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    Vout = Vin * Rf / Riwhere Rf = feedback resistor value and Ri = input resistor value.
     
  5. cambria83

    Thread Starter New Member

    Nov 14, 2007
    3
    0
    I saw an equation similar to this before but I got confused because I don't have any resistor values given to me do I? I think the reason I am so stuck at working this out is because I have only been given those two values! or am I just being really stupid here? :confused:
     
  6. Distort10n

    Active Member

    Dec 25, 2006
    429
    1
    The information you have is incorrect. If it is an inverting attenuator, then the output voltage must be negative because the input voltage is positive. So far, you have give us both positive voltages.
    A non-inverting amplifier can never have a gain less than unity (Av > 1), so the 'negative feedback voltage' a.k.a. output voltage, will never be less than the input voltage.
     
  7. chesart1

    Senior Member

    Jan 23, 2006
    269
    1
    I think the teacher means that the there is a voltage divider between the op amp output and the op amp negative input terminal that sets 4 millivolts at the op amp negative input terminal. The statement is negative feedback, not inverted output.

    I can't be sure because the wording of the problem is vague.
     
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